With rapid spread and wide application of thin film transistor liquid crystal displays (TFT-LCDs), there is an increasing demand for a larger display screen, and thus pixels for display are also becoming larger, which in turn imposes a high requirement on the magnitude of the charging current. How to improve the charging current of the pixels has become critical in the technology.
A typical TFT-LCD comprises a TFT array substrate and a color filter substrate arranged in parallel to each other. Spacers are interposed between the two substrates to provide a space for filling liquid crystal therein. On the TFT array substrate, gate lines and data lines (signal lines) that intersect with each other to define pixels, and at the intersection between a gate line and a data line, there is provided a TFT as a switching element for the corresponding pixel. The TFT comprises a gate electrode connected with the gate line, a source electrode connected with the data line, and a drain electrode connected with a pixel electrode of the corresponding pixel. The voltage applied on the gate electrode over the gate line controls the on/off state of the TFT, that is to say, whether the pixel electrode of the pixel is charged with charging current passing through the TFT channel from the data line.
FIGS. 1 and 2 are diagrams showing a conventional pixel structure of a TFT. As shown in FIG. 2, a bottom gate type TFT comprises a gate electrode 1, which is formed on a substrate 100 and connected with a gate line, a gate insulation layer 14, a semiconductor layer 3, a source electrode 5 and a drain electrode 4, and a passivation layer 15, which are formed on the gate electrode 1 in the order. The semiconductor layer 3 comprises a channel region between the source electrode 5 and the drain electrode 4. A transparent pixel electrode 7 is formed on the passivation layer 15 and connected with the drain electrode 4 via a through hole. When the gate electrode 1 is at a voltage of high level, induced charges are generated in the semiconductor layer (e.g., an amorphous silicon (a-Si) layer) 3 of the TFT, so the channel of the TFT is turned to “on” state, and the charges flow from the drain electrode 4 to the source electrode 5. The transparent pixel electrode 7 is charged in accordance with the magnitude of the voltage signal supplied over a signal line 2 (FIG. 1) to display a desired image.
The magnitude of the charging current passing through the TFT channel can be calculated with the following formula (1-1) and is in direct proportion to the ratio of width to length (W/L) of the channel, wherein W denotes the width of the channel, and L denotes the length of the channel. Therefore, the efficient and popular method is to increase W/L of the channel by modifying the design of the channel structure, so as to increase the charging current of the TFT. A conventional charging channel structure is shown in FIG. 3, wherein W=W1+W2+W3, L=L0 and a U-type channel structure with a single source electrode 5 and a single drain electrode 4 is employed. A large-size TFT-LCD generally needs a larger charging current. However, the conventional charging channel structure may lead to many failures due to the insufficient charging current and thus cannot satisfy the demand for a higher charging current of a TFT-LCD.
The formula of calculating the charging current of a TFT is as follows:
                              I          on                =                  μ          ⁢                                          ⁢                      C            ox                    ⁢                      W            /                          L              ⁡                              [                                                      (                                                                  V                        gs                                            -                                              V                        th                                                              )                                    -                                                            V                      ds                                        2                                                  ]                                              ⁢                                    V              ds                        .                                              (                  1          -          1                )            
The ratio of width to length (W/L) of the conventional channel structure is calculated as follows:
                              W          L                =                                                            W                1                            +                              W                2                            +                              W                3                                                    L              0                                .                                    (                  1          ⁢                      -                    ⁢          2                )            
To increase the W/L of the channel, a conventional method is to adopt a channel structure with a plurality of sources and drains. FIG. 4 shows a diagram of a conventional multi-channel design. As shown in FIG. 5, there are two or more U-type channel structures, and therefore the width of the channel can be enlarged. The W/L of the channel and thus the charging current can be increased. However, because the above design employs two or more sources 5, the aperture ratio of the pixel regions of the LCD decreases and the display brightness and contrast ratio of the TFT-LCD decrease.